AN ISOTOPIC INVESTIGATION OF THE SOURCES OF RECHARGE IN THE COACHELLA VALLEY AQUIFER SYSTEM, RIVERSIDE COUNTY, CALIFORNIA

The Coachella Valley Aquifer is the main source of water for stakeholders in the Coachella Valley, and has been under draw-down conditions since 1936. Presently, Coachella Valley water agencies import ~225,000 Afy of Colorado River water to combat continued draw-down conditions. In this investigation, stable and radioisotopes of water and carbon in approximately 100 samples of combined surface, spring and well waters from the study area are used to contrast and compare the key findings of Tyleys’ 1974, USGS “Analog Model Study of the Ground-Water Basin of the Upper Coachella Valley, California”.

Water isotope data demonstrate that springs located in the San Jacinto and San Bernardino mountains plot along a Local Meteoric Water Line. San Gorgonio and Garnet Hills subbasin groundwater along with Whitewater River and Mission Creek watershed runoff and underflow mix and recharge the Indio subbasin; with very little mixing of groundwater from the Desert Hot Springs subbasin, or imported Colorado River water. Nested and single wells located near the Windy Point recharge facility show shallow wells have water isotope values similar to imported Colorado River water, with deep wells having water isotope values similar to wells and low elevation springs located in the watersheds of the San Gorgonio subbasin and Whitewater River watershed. Two wells proximal but down-gradient from recharge basins have a ~ 40% and 60% mixture of Colorado river water with native groundwater, respectively. Stable water isotopes and radioisotopes in sample waters indicate recharge to the subbasins of the Coachella Valley Aquifer system occurs mainly as winter precipitation in the watersheds of the flanking Santa Rosa, San Jacinto and San Bernardino mountains with limited recharge occurring on the Coachella Valley floor.